CASE NOTES

Intraruminal anthelmintic capsules: risk and reward

Greg Little, Head of Regulatory Affairs & PV, ANZ, Boehringer Ingelheim Animal Health Australia Pty. Ltd. Level 1, 78 Waterloo Road, North Ryde NSW 2113 and Bruce Watt, Central Tablelands Local Land Services, Bathurst

Posted Flock and Herd November 2024

Introduction

Lambing is the most critical period for effective worm control to reduce levels of pasture contamination and production losses. Ewes faced with the additional requirements of lactation and lambs naïve to worms create a period of high infection risk. Intraruminal capsules provide a simple method of minimising this risk and help to maintain healthy, productive sheep. However, appropriate use is key to minimising their potential to contribute to anthelmintic resistance in worms.

Correct administration is also critical to animal safety. Factors such as lack of training, inexperience with sheep handling equipment, lack of manpower, fatigue and high throughput rates have all been associated with maladministration and sheep deaths.

DEVELOPMENT AND BENEFITS OF INTRARUMINAL CAPSULES

In recent trials, intraruminal capsules provided greater protection from worms and reduced pasture contamination compared to either a short-acting pre-lambing treatment, or a short-acting pre-lambing treatment with an additional treatment at lamb marking (Figure 1).

Graph of pasture helmintic contamination

Figure 1. Intraruminal capsules reduced pasture contamination from ewes by 90% compared to an effective short-acting treatment

USING INTRARUMINAL CAPSULES WHEN RESISTANCE IS PRESENT

Research in Western Victoria has confirmed the benefits of using Dynamax Capsules in pre-lambing ewes for worm control, even when challenged with worms resistant to multiple actives¹.

When both abamectin and benzimidazole resistance is present (with a combined efficacy of <95%), some worms may survive, reducing the benefits from capsules and increasing the development of resistance.

However, the simple practice of priming the capsule (drenching with an unrelated active to achieve high efficacy overall) was shown in the trials to improve worm control and lower pasture contamination compared to using a short-acting treatment both at pre-lambing and again at lamb marking (Figure 2).

Graph of capsule efficacy to helmintic resistance

Figure 2. Priming a Dynamax Capsule improves efficacy when resistance is present, and reduced pasture contamination by 49% compared to using a short-acting treatment alone.

INTRARUMINAL CAPSULES AND THE RISK OF DEVELOPING RESISTENCE

Studies reinforce the important role lambs play in managing resistance². Lambs from the intraruminal capsule-treated ewes in this trial helped preserve the population of susceptible worms as they became infected by worms that existed on the paddock prior to the pre-lambing treatment. Susceptible worms held in refugia within the suckling lambs were not exposed to the ewe's intraruminal capsule treatment. Not only were worms kept in refugia but compared to lambs from ewes drenched with a short-acting treatment, the magnitude of the lamb infection was reduced, allowing greater growth rates in lambs from ewes treated with intraruminal capsules.

Studies in ewes / lambs suggest that only moxidectin has an effect on worms in suckling lambs.^{3, 4.}

CASE REPORT: MALADMINISTRATION OF INTRARUMINAL CAPSULES IS A SERIOUS RISK TO THE EWE (AND LAMB)

Maladministration of capsules has led to capsules rupturing the oesophagus leading to infection, inanition and death. Capsule maladministration has also led to capsules lodging in the pharynx, leading to chronic weight loss, inanition and death.

In the case reported here, Dynamax Controlled Release Capsules (capsules) were administered to a large mob of merino ewes in the spring of 2023. Ewes started dying three days after capsule administration and by two weeks after capsule administration 4% of the ewes had died.

Four pregnant merino ewes were necropsied two weeks after capsule administration. All had a capsule lodged in necrotic tissue in or adjacent to the cervical oesophagus (Images 1 and 2).

Image of sheep post-mortem showing a capsule in necrotic neck tissue

Image 1. Wing of capsule protruding from necrotic tissue in the anterior neck in a ewe that died two weeks after capsule administration

Image 2. Capsule lodged in necrotic tissue in the neck in a ewe that died two weeks after capsule administration

Twelve weeks after capsule administration, the authors humanely euthanased and then necropsied another three ewes. These ewes were thin and weak compared to the remainder of the ewes. Two ewes had a capsule lodged in a walled-off pocket adjacent to the oesophagus (Image 3). The third ewe was able to stand and walk but was weak prior to euthanasia. This ewe had a capsule lodged within a pocket in the pharynx but with two thirds of the capsule protruding into the pharynx. This ewe had severe chronic pleuropneumonia of both lung lobes.

An additional ewe, with a lamb at foot was examined in the paddock but was not euthanised. A foreign body consistence with a capsule was palpable in the pharynx (Image 4).

Image of sheep post-mortem showing a capsule in a pocket

Image 3. Capsule lodged within a walled-off pocket beside the anterior oesophagus in a thin ewe nine weeks after capsule administration

Image of sheep post-mortem showing a capsule in the pharynx

Image 4. A foreign body consistence with a capsule was palpable in the pharynx of a thin ewe nine weeks after capsule administration

The property was again visited 17 weeks after capsule administration. Six ewes that were markedly thinner than their cohorts were humanely euthanased and necropsied. Three of these ewes had severe pneumonia without evidence of injury due to capsule maladministration. Three ewes had a capsule lodged in the back of the pharynx protruding into the pharynx (Image 5).

Image of sheep post-mortem showing a capsule in a pocket of the pharynx

Image 5. A thin ewe euthanased and necropsied 17 weeks after capsule administration showing a capsule lodged in a pocket in the caudal pharynx

It is estimated that 15% of ewes died in the 4-5 months after capsule administration.

Discussion

Minimising anthelmintic resistance

While Dynamax Capsules are the only long-acting product that provide dual active worm control, the use of long-acting products increases the risk of developing resistance⁵. However, the risk associated with capsules can be managed.

Two management practices that promote sustainable practice are simple to implement within existing farm management systems. The first is to restrict use of capsules to pre-lambing ewes, allowing their lambs to provide worms in refugia. Lambs are infected from pasture from pre-treatment populations, and due to their immunological naivety, allow further contamination of paddocks with unselected genotypes. The second practice is to use an unrelated active at the time of capsule administration ('priming') to ensure high efficacy against the adult worm population. Priming capsules has been shown to reduce the number of resistant eggs contributed to pasture following treatment.

Minimising the risk of capsule maladministration

Several factors have been associated with maladministration. These include:

New users
High throughput rates / doing the job quickly
Fatigue associated with large daily dosing targets relative to labour numbers
Sheep handling equipment moving sheep into non-standard positions
Use of experienced / confident sheep handlers, but with no capsule application experience (especially outside traditional capsule using regions of WA/SA/Vic)
Sheep size or maturity (as per label). Increasing ewe size prevents straddling the ewe to restrain movement as per label instructions.

Diet does not appear to be a risk factor.

To address the major factors and reduce risk:

A Boehringer Ingelheim (BI) field team can provide on-farm training to users.
The BI field team can provide on-farm train-the-trainer sessions to inform rural merchandise staff.
Online training modules are used to ensure compliance to agreements to sell capsules. Training is focused on the need to identify the risk of administration and ensure those who are administering them understand this risk.
Training is completed using sheep straddled in a race prior to use of handlers.

However, challenges to addressing these factors remain. Often, the purchaser of the capsules is not the person administering them, either employing contractors or other employees to perform the task. Additionally, administrators can be overconfident in their ability or do not understand the risk, and therefore do not request training. Training of users in capsule administration using sheep handling equipment remains a challenge due the variety of devices on the market.

References

Kelly GA and Besier B (2021) The effect of co-administration of an unrelated short-acting treatment and controlled-release capsule on worm control in the presence of anthelmintic resistance and its timing. In Proceedings of the 28^th Conference of the World Association for the Advancement of Veterinary Parasitology, Dublin
Kelly GA, Nielson R & Doherty M (2017) The effect of a controlled release capsule on worm control measures and production in sheep in a high rainfall region of Australia. In Proceedings of the 9th International Sheep Veterinary Congress
Dever ML & Kahn LP (2015) Decline in faecal worm egg counts in lambs suckling ewes treated with lipophilic anthelmintics: Implications for hastening development of anthelmintic resistance Veterinary Parasitology 209:229-234
Leathwick DM, Miller CM & Fraser, K (2015) Selection for anthelmintic resistant Teladorsagia circumcincta in pre-weaned lambs by treating their dams with long-acting moxidectin injection International Journal for Parasitology: Drugs and Drug Resistance 5:209-214
Dobson RJ, Lejambre L, & Gill JH (1996) Management of anthelmintic resistance: inheritance of resistance and selection with persistent drugs International Journal for Parasitology 26(8-9):993-1000